This application claims the priority of Application No. H11-209703, filed Jul. 23, 1999 in Japan, the subject matter of which is incorporated herein by reference.
The present invention relates to photocouplers and connecting systems using photocouplers, such as power supply system, terminal switching system, etc.
A photocoupler may be provided in a connecting line to open and close the line selectively. Photocouplers have been used for a variety of switching systems, such as a switching device provided between power supply and electric device, a switching device for switching communication terminals to be connected to a communication line, and to a telephone switchboard. In general, a photocoupler includes an output unit provided with a power device, which controls switching operation of connecting lines. Such a power device may be a MOSFET, photodiode, phototransistor, photo-Darlington, photo-thyristor or the like.
According to a conventional photocoupler, the power device in the output unit becomes equivalent to a resistance in circuit when the connecting lines are closed. The value of the resistance increases in response to Joule heat resulting form an electric current flowing through the resistance. Therefore, a large amount of load is applied to the power device when a large amount of current flows through the connecting lines. If over current (excess current), which is the current exceeds a rated value (surge current withstand) of the power device, flows through the connecting lines, the power device would be damaged, and in many cases, a short circuit is made. As a result, the electric current keeps flowing through the connecting lines.
As described above, when the photocoupler breaks down or be damaged while the connecting lines are still closed (connected), the whole system connected with the photocoupler may be damaged as well.
Accordingly, an object of the present invention is to provide a photocoupler which keeps being opened when a power device is damaged in response to over current (excess current).
Additional objects, advantages and novel features of the present invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
According to a first aspect of the present invention, a photocoupler includes a light emitting unit which emits input light; a first path connecting a first terminal and a second terminal; a switching circuit provided in the first path to open and close the first path in response to the input light; and a circuit breaker provided in the first path to disconnect the first path in response to over current (excess current). When an over current (excess current) is supplied to the first path, the photocoupler becomes out of order under a condition that the first path keeps being opened (disconnected). The switching circuit may include a photo detector, which generates an electric signal in response to the input light; and a switching element which opens and closes the first path in response to the electric signal, supplied from the photo detector.
The circuit breaker may be provided between two of the switching elements. When more than three switching elements are used, the circuit breaker would be arranged between two of them. A back-up (extra) terminal can be provided between adjacent two switching elements so that the back-up terminal is used instead of the first or second terminal when one of the switching elements disconnects the first path.
The switching element may be a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) so that a MOS output type of photocoupler is made. Further, the circuit breaker and photo detector may be fabricated to be in a single chip apart from the switching element.
The switching circuit may include a discharge controller provided in a second path, connecting the photo detector and switching element, to control the discharge condition of the switching element. The discharge controller may be designed to discharge electric charge accumulated in the switching element so that the response speed of the switching element is improved.
The discharge controller may be driven in response to the input light. The discharge controller can be driven concurrently with the photo detector. As a result, discharge controlling operation of the switching element can be carried out automatically.
The switching element may be a semiconductor element that is one selected from a group composed of photodiode, phototransistor, photo-Darlington and photo-thyristor. The circuit breaker, photo detector and discharge controller may be fabricated to be in a single chip apart from the switching element.
The circuit breaker may include a section that is melted and cut off in response to over current (excess current) so that the first path is disconnected reliably.
The circuit breaker may include plurality fuses which are melted and cut off in response to over current (excess current) flowing through the first path. The fuses may be connected in parallel in the first path. According to this aspect of the invention, the fuses are not cut off all together but cut off one by one, so that load applied to the switching circuit when the first path is disconnected is decreased.
The fuses may include a first fuse and a second fuse, in which the second fuse has a smaller cross-section area than the first fuse. The first and second fuses are connected in series. Joule heat is generated more at the second fuse, so that the first path is disconnected reliably.
The section included in the circuit breaker may be made of an aluminum wire so that resistance value of the first path can be prevented from being increased.
The photocoupler may include plural circuit breakers provided in series in the first path. A back-up (extra) terminal can be provided between adjacent two circuit breakers so that the back-up terminal is used instead of the first or second terminal when one of the circuit breakers disconnects the first path.